(1) Field of the Invention
The present invention concerns a transflective liquid crystal display device and, more specifically, it relates to a technique which is effective to the application for IPS system transflective liquid crystal display devices.
(2) Description for the Related Art
A transflective liquid crystal display device having a transmission portion and a reflection portion in a 1 sub-pixel has been used as a display for use in portable equipments.
For the transflective liquid crystal display device, a vertical electric field system for driving liquid crystals by applying an electric field to liquid crystals put between a pair of substrates in a direction vertical to the substrate plane of the pair of substrates has been used. Further, for mating characteristics between the transmission portion and the reflection portion, a step is formed between the transmission portion and the reflection portion and, further, a phase difference plate is disposed between the polarization plate and a liquid crystal layer.
As the liquid crystal display device, an IPS system liquid crystal display device has been known and, in the IPS system liquid crystal display device, a pixel electrode (PIX) and a counter electrode (CT) are formed on one identical substrate and an electric field is applied therebetween to rotate liquid crystals in a substrate plane thereby conducting bright/dark control. Accordingly, it has a feature that the contrast density of displayed images is not reversed upon observing a screen from an oblique direction. For taking advantage of the feature, it has been proposed to constitute a transflective liquid crystal display device by using the IPS system liquid crystal display device, for example, in Japanese Patent Laid-Open Publication No. 2003-344837.
Usually, the IPS system transparent liquid crystal display device is normally black. Accordingly, in a case of constituting the transflective liquid crystal display device by using the IPS system liquid crystal display device in a case where a phase difference plate is not provided, there is a problem that the reflection portion becomes normally white in a case where the transmission portion is a normally black and the bright/dark state is reversed between the transmission portion and the reflection portion as described also in the patent document described above.
For solving the problem described above the present applicant has already filed a patent application regarding a transflective liquid crystal display device having a novel pixel structure (refer to Japanese Patent Application No. 2005-322049).
In the transflective liquid crystal display device of the former patent application, as a pixel structure for each of sub-pixels, counter electrodes are made independent between the transmission portion and the reflection portion respectively relative to a pixel electrode which is in common with the transparent portion and the reflection portion and different reference voltages (counter voltage or common voltage) are applied respectively thereby preventing reversal of bright/dark state between the transmission portion and the reflection portion.
Further, in the transflective liquid crystal display device already filed, the transmission portion has a normally black characteristic (black display in a state of not applying voltage) and the reflection portion has a normally white characteristic (white display in a state of not applying the voltage).